"Todd C. Miller":

> Do you know which format other versions of tar use by default?

bsdtar (libarchive, FreeBSD) defaults to "restricted pax":
    The libarchive library can also write pax archives in which it
    attempts to suppress the extended attributes entry whenever
    possible.  The result will be identical to a ustar archive unless
    the extended attributes entry is required to store a long file
    name, long linkname, extended ACL, file flags, or if any of the
    standard ustar data (user name, group name, UID, GID, etc) cannot
    be fully represented in the ustar header.  In all cases, the
    result can be dearchived by any program that can read POSIX-
    compliant pax interchange format archives.  Programs that
    correctly read ustar format (see below) will also be able to read
    this format; any extended attributes will be extracted as
    separate files stored in PaxHeader directories.

GNU tar defaults to "gnu".  ("posix" aka "pax" will be the default
format for future versions.)  As far as I know, backwards compatibility
is similar to that of restricted pax from above.  The compile time
defaults are listed at the end of the "gtar --help" output.

-- 
Christian "naddy" Weisgerber                          naddy@mips.inka.de